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The mushrooms are simply the fruiting bodies of mycelium present in the compost. To check whether the plant is revivable or not, scrape back a little of the bark or outer covering on the main stem with your fingernail in a few places, particularly towards the lower part of the stem. If beneath is brown and dry, it's dead - if it looks moist and greenish, then turn it out of the pot, shake the compost off, rinse the roots in water, and repot using fresh compost in a clean pot.

There seems to be an issue with watering and/or humidity. If the brown leaves on the 'live' plant went soggy and then brown, overwatering. If they went dry and crispy and brown, underwatering.